An electrolytic capacitor refers to a capacitor employing as an electrode a so-called valve metal such as aluminum, tantalum, or niobium and including as a dielectric an oxide film layer formed through anode oxidation.
An aluminum electrolytic capacitor generally has a structure shown in FIGS. 1 and 2. A capacitor element 6 is formed by rolling an anode foil 1 and a cathode foil 2 each subjected to etching treatment and oxide film formation treatment through a separator 3. Then, the capacitor element is impregnated with an electrolytic solution, and included in a cylindrical outer case 8 having a closed-end.
Next, anode and cathode extraction leads 4 and 5 are inserted through and extracted from through-holes formed on an elastic sealing body 7. The sealing body (elastic sealing body) 7 formed of a material having elasticity is attached to an open end of the outer case, to thereby provide a structure sealed through drawing.
Another aluminum electrolytic capacitor has a structure shown in FIGS. 3 and 4. The capacitor element 6 is formed by rolling the anode foil 1 and the cathode foil 2 each subjected to etching treatment and oxide film formation treatment through the separator 3. Then, the capacitor element is impregnated with an electrolytic solution, and included in the cylindrical outer case 8 having a closed-end. A sealing body 9 is attached to an open end of the outer case 8, to thereby provide a structure sealed through drawing. The aluminum electrolytic capacitor may include an element fixing agent 17 for fixing the capacitor element 6 in the outer case 8.
An anode terminal 13 and a cathode terminal 14 are formed on an outer end surface of the sealing body 9, and lower ends of the terminals 13 and 14 as an anode internal terminal 15 and a cathode internal terminal 16 are respectively electrically connected to an anode tab terminal 11 and a cathode tab terminal 12, which are extracted from the capacitor element 6.
The anode tab terminal 11 to be used is subjected to oxide film formation treatment, but the cathode tab terminal 12 to be used is not subjected to oxide film formation treatment.
Each of the tab terminals 11 and 12 employs an aluminum foil subjected to no surface processing.
With reduction in size and thickness of electronic components and progress in high density surface mount technology, a chip shape has been required for an aluminum electrolytic capacitor, and a chip aluminum electrolytic capacitor has a structure shown in FIG. 5.
The capacitor element 6 is formed by rolling an anode foil and a cathode foil each subjected to etching treatment and oxide film formation treatment through a separator. Then, the capacitor element is impregnated with an electrolytic solution, and included in the cylindrical outer case 8 having a closed-end. An open end is sealed by using the elastic sealing body 7, to thereby form an aluminum electrolytic capacitor.
The aluminum electrolytic capacitor is arranged so as to be in direct contact with an extraction end surface of a lead terminal 18, and an insulating sheet 19 provided with a through-hole allowing the lead terminal 18 to pass through is attached, to thereby form a structure stably attached to a substrate.
The aluminum electrolytic capacitors each include a separator impregnated with an electrolytic solution between an anode foil and a cathode foil. The electrolytic solution functions as a true cathode, and has such a feature that the electrolytic solution having oxide film formation ability repairs an oxide film undergoing electrical breakdown due to electrical stress, mechanical stress, or the like. The electrolytic solution is therefore an important component providing a large effect in properties of the aluminum electrolytic capacitor.
Conventionally, there is known an electrolytic solution having high electric conductivity, mainly containing γ-butyrolactone as a solvent, and containing a tetraalkyl quaternary ammonium salt of a carboxylic acid such as phthalic acid or maleic acid as a solute to be used in an aluminum electrolytic capacitor having low impedance in a high frequency region (see JP-A-62-145713, for example).
However, the quaternary ammonium salt-based electrolytic solution has low reliability because its base component may ooze from a cathode sealed part.
For avoiding this ooze problem and satisfying low impedance required for an electrolytic capacitor, there is known a so-called amidine-based electrolytic solution containing as an electrolyte a quaternary carboxylic acid salt of a compound having an alkyl-substituted amidine group (see WO95/15572, for example).
The amidine-based electrolytic solution has a better effect of suppressing ooze of the electrolytic solution, but has comparable electric conductivity compared with those of a tetraalkyl quaternary ammonium salt. In general, the amidine-based electrolytic solution has a withstand voltage of about 50 V and can only be used for an aluminum electrolytic capacitor having a rated voltage of 35 V or less. The amidine-based electrolytic solution must have reduced solute concentration and significantly reduced electric conductivity, in order to be used for an aluminum electrolytic capacitor having a rated voltage of 50 V or more.
With recent reduction in size, improvement in performance, and increase in use temperature of electronic devices, an aluminum electrolytic capacitor is required to satisfy low energy loss, good impedance property in a wide temperature range, high withstand voltage property, and long-life property. However, no aluminum electrolytic capacitor has been realized to satisfy such properties.
In view of the circumstances described above, a compound having high withstand voltage and high electric conductivity is required as an electrolyte for an electrolytic solution to be used in an aluminum electrolytic capacitor.
An ionic liquid, which is a liquid at room temperature, has high electric conductivity and thus is probably effectively used as an electrolyte. However, an imidazolium salt or a pyridinium salt of a tetrafluoroboric acid anion, bis(trifluoromethane sulfonyl)imide anion, or the like contains a fluorine ion. Thus, such an imidazolium salt or a pyridinium salt has problems such as corrosion of an electrode, and is not a compound to be used for the aluminum electrolytic capacitor.
Thermal properties, viscosity, and qualitative electric stability of a dicyanoamide salt of N-alkyl-N-methyl pyrrolidium or 1-alkyl-3-methyl imidazolium as a non-fluorine-based ionic liquid are studied, and there is disclosed that such a dicyanoamide salt is effective as an ionic liquid (room temperature molten salt) having low viscosity (see Douglas R. MacFarlane et al., Chem. Commun., 2001, p. 1430-1431 and US2004/0002002, for example).
The above documents disclose a technique of applying an electrolytic solution containing an anion having a cyano group such as a dicyanoamide ion to an electrolytic solution material of an electrochemical device such as a lithium secondary battery.